Nucleoside analogues have been developed as antiviral and anticancer agents. Nucleotide kinases phosphorylate nucleosides to their corresponding 5′-monophophates which are further converted into their di- and tri-phosphates by cellular nucleotide kinases.
It is now known that some nucleosides are weakly active simply because they cannot be efficiently phosphorylated by kinases or are not substrates of kinases at all, as evidenced by that some inactive nucleosides, when converted chemically to triphosphates, become potently active against certain viruses in vitro. Nucleoside phosphates (nucleotides) per se cannot be used as drugs often because they are de-phosphorylated by membrane nucleotides and/or other hydrolases before entering the cells or are too polar to enter the cells. To improve biological activity of nucleosides, their phosphate prodrugs have been intensively studied because they can potentially bypass the rate-limiting first step of phosphorylation. Recently, phosphoramidate prodrug approach has been proved to be an effective method to convert biologically inactive nucleosides to active nucleoside mono-phosphate bypassing the rate-limiting first step of phosphorylation (J. Med. Chem., 50 (22), 5463-5470, 2007). Nucleoside phosphoramidate has been reported to efficiently deliver nucleoside 5′-monophosphate into liver (WO 2008/121634; WO 2008/082601 and WO 2008/082602). In recent years, there are a number of patent applications disclosing utilization of the phosphoramidates as prodrugs to deliver nucleoside monophosphates to tissues, in particular to the liver (U.S. Pat. No. 6,455,513, WO 2009/052050, WO 2008/121634, WO 2008/0833101, WO 2008/062206, WO 2007/002931, WO 2008/085508, WO 2007/095269, WO 2006/012078, WO 2006/100439). The nucleoside monophosphates can be further phosphorylated to di-, and then biologically active triphosphate.
However, the above-mentioned phosphoramidate approaches based on McGuigan's technology (U.S. Pat. No. 6,455,513) have various limitations due to potential neurotoxicity and liver and kidney damages caused by phenol released from prodrugs (Carcinogenesis 1993, 14: 2477-2486; Mutat. Res. 1991, 249, 1: 201-209).
Mcguigan's phosphoramidate of nucleoside usually can demonstrate maximum biological activity in cell line assays because it can release nucleoside or nucleotide quickly in the cells. It was reported that phosphoramidate prodrug of d4T could not be detected in plasma after oral administration (Drug Metab. Dispos. 2001, 29, 1035). Phosphoramidate is stable in gastric fluid and may be absorbed in the stomach. On the other hand, phosphoramidate may decompose readily in intestinal fluid to ala-d4T-MP. This metabolite may be absorbed in the intestine and further metabolized to yield nucleoside d4T. Another possibility is that this metabolite (ala-d4T-MP) may not be absorbed efficiently in intestine due to its polar nature. Therefore, bioavailability of this type of esterase sensitive phosphoramidate prodrugs is relatively low probably due to its hydrolysis catalyzed by esterase. For example, bioavailability of GS-7340, an isopropylalanyl monoamidate phenyl monoester of tenofovir, was 17% in male beagle dogs (Gilead Sciences, Antimicrob. Agents Chemother. 2005, 49, 1898).
Diamide prodrugs of nucleoside phosphonates have also been investigated, and in certain cases this approach appears to improve the potency and/or pharmacokinetic profile. However, only very limited research is available on the application of phosphoric diamides to nucleoside (anti-HIV agents, FLT, AZT; Polish J. Chem. 1993, 67, 755; Drug Design and Discovery 1995, 13, 43; Antiviral Chem. Chemother. 1992, 3, 107; 1991, 2, 35; 1995, 6, 50). PMEA diamide prodrug could not provide satisfactory bioavailability of the parent drugs probably due to its higher polarity than Tenofovir phosphonoamidate (J. Med. Chem. 2008, 51, 4331; Antimicrob. Agents Chemother. 2005, 49, 1898).
Efforts to search for phosphonate prodrugs that would be cleaved by an esterase independent mechanism have led to the discovery of two classes of prodrugs that have advanced into human clinical trials, namely bisphenyl esters and HepDirect prodrugs (J. Med. Chem. 1994, 37, 498; J. Am. Chem. Soc. 2004, 126, 5154; J. Pharmacol. Exp. Ther. 2005, 312, 554). Bisbenzyl esters have also been investigated but the simple unsubstituted benzyl ester is cleaved too slowly to be of use as a prodrug (Bioorg. Med. Chem. Lett. 2007, 17, 3412). Erion et al disclosed that cyclic phosphate or phosphonate prodrugs which are stable in the presence of esterase can enhance liver specific drug delivery (Erion, M et al U.S. Pat. No. 7,303,739 and reference thereof). Erion's prodrugs are activated by P450 in the liver. However, clinical application of this approach may be limited by potentially adverse side effects caused by aromatic metabolites, and the efficiency of releasing bioactive phosphate or phosphonate is rather dubious.
Recently, bis[(para-methoxy)benzyl]phosphonate prodrug was reported to have improved stability and enhanced cell penetration (Bioorg. Med. Chem. Lett. 2007, 17, 3412). However, no further details were presented. Since it is known that simple benzyl phosphate prodrug and O-benzyl phosphoramidate are too stable to release active nucleoside phosphate, new prodrug forms of nucleoside and nucleotide compounds are still being actively pursued.